The present invention generally relates to an atrial defibrillator for applying cardioverting electrical energy to the atria of a human heart in need of cardioversion. The present invention is more particularly directed to a fully automatic implantable atrial defibrillator which exhibits improved safety by reducing the potential risk of induced ventricular fibrillation which may result from the mistimed delivery of cardioverting electrical energy to the atria of the heart. More specifically, the atrial defibrillator of the present invention guards against applying cardioverting electrical energy to the atria of the heart under conditions believed to contribute to induced ventricular fibrillation.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life-threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience palpitations of the heart and may even experience dizziness or even loss of consciousness.
Atrial fibrillation occurs suddenly and many times can only be corrected by a discharge of electrical energy to the heart through the skin of the patient by way of an external defibrillator of the type well known in the art. This treatment is commonly referred to as synchronized cardioversion and, as its name implies, involves applying cardioverting or defibrillating electrical energy to the heart in synchronism with a detected depolarization activation wave (R wave) of the heart. The treatment is very painful and, unfortunately, most often only results in temporary relief for patients, lasting but a few weeks.
Drugs are available for reducing the incidence of atrial fibrillation. However, these drugs have many side effects and many patients are resistant to them which greatly reduces their therapeutic effect.
Implantable atrial defibrillators have been proposed to provide relief to patients suffering from occurrences of atrial fibrillation. Unfortunately, to the detriment of such patients, none of these atrial defibrillators have become a commercial reality.
Implantable atrial defibrillators proposed in the past have exhibited a number of disadvantages which probably have been the cause of these defibrillators failing to become a commercial reality. Two such proposed defibrillators, although represented as being implantable, were not fully automatic, requiring human interaction for cardioverting or defibrillating the heart. Both of these defibrillators require the patient to recognize the symptoms of atrial fibrillation with one defibrillator requiring a visit to a physician to activate the defibrillator and the other defibrillator requiring the patient to activate the defibrillator from external to the patient's skin with a magnet.
Improved atrial defibrillators and lead systems which exhibit both automatic operation and improved safety are fully described in U.S. Pat. No. 5,282,837, issued Feb. 1, 1994 in the names of John M. Adams and Clifton A. Alferness for "Improved Atrial Defibrillator and Method" and U.S. application Ser. No. 07/856,514, filed Mar. 24, 1992 in the in the names of John M. Adams, Clifton A. Alferness, and Paul E. Kreyenhagen for "Improved Atrial Defibrillator, Lead Systems, and Method", now U.S. Pat No. 5,433,729, which patents are assigned to the assignee of the present invention and incorporated herein by reference. As disclosed in the aforementioned referenced patents, synchronizing the delivery of the defibrillating or cardioverting electrical energy to the atria with a ventricular electrical activation (R wave) of the heart is important to prevent induced ventricular fibrillation. Ventricular fibrillation is a fatal arrhythmia which can be caused by electrical energy being delivered to the heart at the wrong time in the cardiac cycle, such as during the T wave of the cycle. The atrial defibrillators of the aforementioned referenced applications exhibit improved safety from inducing ventricular fibrillation by sensing ventricular activations of the heart in a manner which avoids detecting noise as ventricular electrical activations for generating reliable synchronization signals. Hence, these implantable atrial defibrillators, by providing such noise immunity in R wave detection, assure reliable synchronization.
The aforementioned U.S. Pat. No. 5,282,837 describes non-coincident sensing of an electrical activation such as an R wave at two different areas of the heart to provide a reliable indication that the sensed electrical activation is a real or legitimate electrical activation and not noise or other interference. Non-coincidentally sensed electrical activations, in accordance with the teachings of U.S. Pat. No. 5,282,837, are considered to be legitimate electrical activations. Others are considered to be noise or other interference. The non-coincidentally sensed electrical activation thus can be relied upon for synchronizing the delivery of a defibrillating or cardioverting electrical pulse to the atria.
It has further been observed that during episodes of atrial fibrillation, the cardiac rate increases to a high rate and/or becomes extremely variable. At high cardiac rates, the R wave of each cardiac cycle becomes closely spaced to the T wave of the immediately preceding cardiac cycle. This may lead to a condition known in the art as an "R on T" condition, which is believed to contribute to induced ventricular fibrillation if the atria are cardioverted in synchronism with an R wave close to a T wave.
An atrial defibrillator and method which greatly reduces the risk of inducing ventricular fibrillation during atrial cardioversion or defibrillation by avoiding applying the cardioverting electrical energy to the atria at those instances when increased vulnerability to ventricular fibrillation may be present is described in U.S. Pat. No. 5,207,219, issued May 4, 1993 to John M. Adams, Clifton A. Alferness, Kenneth R. Infinger, and Joseph M. Bocek, which patent is assigned to the assignee of the present invention and incorporated herein by reference. As described in the referenced patent, this is accomplished by interval timing prior to applying the cardioverting or defibrillating electrical energy. The time interval between immediately successive R waves is timed by an interval timer and the cardioverting or defibrillating electrical energy is only applied when the interval timer times an interval which is greater than a preselected minimum interval. This provides protection from the increased vulnerability to ventricular fibrillation resulting from a high cardiac rate.
U.S. Pat. No. 5,207,219 contemplates, in accordance with a preferred embodiment, the resetting of the interval timer responsive to R waves detected in the right ventricle of the heart. However, while this is generally successful, it has been learned that R waves detected in the right ventricle during atrial fibrillation have highly variable amplitudes. Hence, as an added measure of safety, it would be desirable to sense or detect R waves at more than one location of the heart and reset the interval timer responsive to an R wave detected at any one of the R wave detection locations. This will assure reliable timing initiation by the interval timer notwithstanding the variability of the amplitudes of depolarization activation waves sensed at any one location of the heart. Hence, while an R wave may be missed due to an extremely low amplitude at one location of the heart, it will still be detected at another location for resetting the interval timer.
The atrial defibrillator and method of the present invention greatly reduces the risk of inducing ventricular fibrillation during atrial cardioversion or defibrillation by avoiding applying cardioverting electrical energy to the atria at those instances when increased vulnerability to ventricular fibrillation may be present. As will be seen hereinafter, this is accomplished by interval timing prior to applying the cardioverting or defibrillating electrical energy. The time interval between immediately successive R waves is timed and the cardioverting or defibrillating electrical energy is applied only when a timed interval is greater than a preselected minimum interval. Timing is reset in response to sensing a depolarization activation wave in one of a first area and a second area of the heart to assure that all R waves are used for resetting the interval timing.